The endoplasmic reticulurn (ER) is a densely folded stack of unit
membranes, often with the appearance of being wrapped in concentric
layers around the nucleus. The membranes of the ER have an "inside"
and an "outside" and enormous surface area. One side of these folded
ER membranes-the side facing the cytoplasm-is liberally peppered
with ribosomes for protein synthesis. Other ribosomes are found
floating loose in the cytoplasm.
Although the details are hard to see in any one micrograph, serial
sections reveal that the highly folded ER actually is continuous
with the outer cell membrane. In reality it is a folded membrane
that encloses a labyrinth of deep cavities inside the body of the
cell. The side of the ER that lacks ribosomes is topologically connected
with the exterior of the cell, and the ribosome-containing side
is everywhere in contact with the cytoplasm.
The ER also is connected to the nuclear membrane and the Golgi complexes.
It provides channels for access from the cell surface to deep within
its interior, and an exit route for small molecules produced in
the cell. In addition to protein synthesis, the inner surface of
the ER is the place where fatty acids are esterified to fats for
storage in fat globules in the cytoplasm, where phospholipids and
cholesterol are synthesized for use in membranes, and where sugars
are polymerized to mucopolysaccharides for secretion between cells.